In recent years, since printers and copying machines have been requested to have a higher process speed, higher stability, and a further reduced size, functions of components thereof have been improved. Concomitant with the above improvement in functions of the components, the number thereof is increased, and hence it is now requested to reduce the number of components. In order to obtain stable image density and hue in an electrophotographic method, it is necessary to always form predetermined development conditions in a developing process. However, if the charge amount of toner is not stable, a considerable load is applied to a system for controlling the developability such that, for example, developing bias conditions must be optimized each time, and as a result, in many cases, the size of a device is increased, and/or the manufacturing cost is increased. In order to reduce the load as described above, an improvement in stability of the charge amount of toner, and in particular, an improvement in stability of the charge amount of toner against the change in temperature and humidity have been required.
Many techniques have been proposed to improve the environmental stability of the toner charge amount. Among the techniques, controls using a charge control agent have been the mainstream technique, and for example, a toner containing a calixarene compound, a toner using an iron-containing azo dye, and a toner using an organic boron compound have been proposed (for example, see Patent Documents 1 to 4). However, the toners described above are not sufficient in terms of the toner charge amount and the charge rise characteristic of the toner with the change in temperature and humidity environment. For example, the change in image density occurs in printing, and in particular, in high temperature and high humidity environment, inconveniences, such as image fogging, caused by non-uniform charge amount distribution occur in some cases. Furthermore, in order to obtain the hue stability of image, color mixability of the toner is also important, and in particular, at a high-light portion, the transparency of the toner is required. In addition, as a colorant used for the toner, highly stable pigments have been mainly used in consideration of discoloration and the like. Various techniques for dispersing a pigment in the toner have been proposed. Among those techniques, many proposals in which a polar resin is added have been mainly made, and in more particular, polyester-based charge control agents obtained by polycondensation of monomers containing a sulfonic acid (sulfonic salt) have been proposed (for example, see Patent Documents 5 and 6). According to these proposals, it is believed that since a charge control resin is a polyester resin, the compatibility thereof with a polyester-based binder resin and the dispersibility of pigment are improved. However, in practice, the dispersibility of the pigment in the binder resin is not sufficiently improved by simply changing the composition of the charge control agent, and hence, further improvement has been desired.